Thermal ink jet printers are well known in the art and are illustrated, for example, in U.S. Pat. Nos. 4,490,728, 4,313,124 and 4,296,421. In such printers, a thin film resistor in an ink drop generator is heated and causes sudden vaporization of a small portion of the ink. The rapidly expanding ink vapor displaces ink from a nozzle, causing drop ejection. The ejected drops collect onto a page to form printed characters.
The utility of such printers has been inhibited because of the difficulty involved in achieving gray scale printing.
U.S. Pat. No. 4,503,444 proposes one technique for achieving gray scale printing. In the disclosed system, the thin film firing resistor is provided with several electrical firing pulses in rapid succession (termed a "packet"). The individual droplets formed by the pulses merge in flight to create a single drop. The drop volume, and thus the resulting gray scale, can be varied by varying the number of pulses in the packet.
In an alternative system, disclosed in U.S. Pat. No. 4,746,935, an ink jet pen is provided with three drop generators which produce drops of different volumes, weighted in binary relationship. By firing various combinations of the drop generators at a given pixel on the printing medium, eight levels of print density can be achieved.
In still another system, disclosed in copending application Ser. No. 899,447 of Taub and assigned to the present assignee, ink jet gray scaling is achieved by merging ink droplets on the printing medium while still wet. The rate at which droplets are ejected is varied to vary the print density.
The foregoing techniques all rely on the convergence of a plurality of droplets, either in flight or on the printing medium, to yield variations in gray scale. While these techniques achieve the desired results, they do so with certain attendant drawbacks, such as accelerated burnout of the firing resistor due to electromigration.
Other techniques for achieving gray scaling have been proposed which rely on the ejection of ink droplets of varying volumes, such as by varying the amplitudes or durations of the firing pulses. However, these techniques have generally not met with success.
It is an object of the present invention to provide a gray scale printing technique that does not rely on the firing of multiple droplets to achieve gray scaling.
It is a more particular object of the present invention to vary printed gray scale by varying the volume of droplets ejected from the pen.
It is still another more particular object of the present invention to vary printed gray scale by preceding the firing pulse with a warming pulse to warm the ink in accordance with the degree of gray scaling desired.
According to one embodiment of the present invention, the firing resistor in an ink jet pen is driven by two signals in sequence. The first signal is a warming signal, tailored to transfer a desired quantity of thermal energy to the ink in the firing nozzle. The second signal is a firing pulse tailored to vaporize ink adjacent the resistor and thereby eject a bubble of ink from the nozzle. The warming pulse serves to warm the ink in the nozzle and thereby increase the volume of the vapor bubble produced by the firing pulse. By varying the characteristics of the warming pulse, the droplets ejected by the firing pulse can be varied in volume, thereby effecting gray scale printing.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.